A wide array of human diseases could potentially be treated by genetic manipulation of hematopoietic stem cells. However, while transfer of genetic material into murine hematopoietic progenitor cells is routinely efficiently achieved, gene transfer into stem cells of large animal models and humans has thus far proven to be difficult and inefficient. The aim of this project is to gain insights into the biology of the hematopoietic stem cell by using in vitro and in vivo animal models of human hematopoiesis and to improve the efficiency of gene transfer/correction of this rare and elusive target cell. Human hematopoietic progenitors are obtained from cord blood or bone marrow of volunteer donors, cultured and genetically engineered in vitro, and then allowed to differentiate into mature cell lineages in vitro (CFU or LTC-IC assays) or in vivo, using mouse or sheep animal models.